Spider silk proteins and methods for producing spider silk proteins

ABSTRACT

The invention provides an isolated major ampullate spidroin protein, which consists of from 150 to 420 amino acid residues and is defined by the formula REP-CT. REP is a repetitive, N-terminally derived protein fragment having from 80 to 300 amino acid residues. CT is a C-terminally derived protein fragment having from 70 to 120 amino acid residues. The invention further provides an isolated fusion protein consisting of a first protein fragment, which is a major ampullate spidroin protein, and a second protein fragment comprising a fusion partner and a cleavage agent recognition site. The first protein fragment is coupled via said cleavage agent recognition site to the fusion partner. The invention also provides a method of producing a major ampullate spidroin protein and polymers thereof.

This application is a Continuation of application Ser. No. 13/598,119,filed on Aug. 29, 2012, which is a Continuation of application Ser. No.13/441,118, filed Apr. 6, 2012, now U.S. Pat. No. 8,278,416, which is aContinuation of application Ser. No. 12/087,289 filed Jun. 30, 2008, nowU.S. Pat. No. 8,173,772, which is the National phase of PCTInternational Application No. PCT/SE2006/001505 filed on Dec. 28, 2006.This application also claims priority to Patent Application No.0502932-7 filed in Sweden on Dec. 30, 2005. All of the aboveapplications are hereby expressly incorporated by reference into thepresent application.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to the field of recombinant production ofproteins. More specifically, the present invention is concerned withrecombinant production of spider silk proteins. The present inventionprovides novel isolated major ampullate spidroin proteins and majorampullate spidroin fusion proteins, as well as methods and polynucleicacid molecules for producing such proteins. There is also providedpolymers of the major ampullate spidroin proteins and methods forproducing such polymers.

BACKGROUND OF THE INVENTION

Spider silks are nature's high-performance polymers, obtainingextraordinary toughness due to a combination of strength and elasticity.Up to seven specialized glands exist in spiders, which produce a varietyof silk fiber types with different mechanical properties and functions.Dragline silk, produced by the major ampullate gland, is the toughestfiber, and on a weight basis it outperforms man-made materials, such ashigh tensile steel and Kevlar. The properties of dragline silk areattractive in development of new materials for medical or technicalpurposes.

Dragline silk consists of two main polypeptides, mostly referred to asmajor ampullate spidroin (MaSp) 1 and 2, but to ADF-3 and ADF-4 inAraneus diadematus. These proteins have apparent molecular masses in therange of 200-720 kDa, depending on sample age and conditions ofanalysis, but no full-length dragline spider silk gene has yet beenreported. The properties of dragline silk polypeptides are discussed inHuemmerich, D. et al. Novel assembly properties of recombinant spiderdragline silk proteins. Curr. Biol. 14, 2070-2074 (2004). The knowndragline silk spidroins are composed of highly iterated blocks ofalternating alanine-rich segments, forming crystalline β-sheets in thefiber, and glycine-rich segments which are more flexible and mainly lackordered structure. The C-terminal region is non-repetitive, highlyconserved between species, and adopts α-helical conformation. TheN-terminal region of dragline silk proteins has not been characterizeduntil very recently, revealing an N-terminal domain that is highlyconserved between different spidroins, and also between different spiderspecies (Rising, A. et al. N-terminal nonrepetitive domain common todragline, flagelliform, and cylindriform spider silk proteins.Biomacromolecules 7, 3120-3124 (2006)).

The mechanical properties of dragline silk varies between species;Euprosthenops sp dragline silk is stiffer, stronger (requires more forceto break) and less extendible than dragline silk from e.g. Araneusdiadematus or Nephila clavipes. Dragline silk from Euprosthenops spappears to have a greater proportion of crystalline β-sheet structurethan dragline silk from Araneus diadematus, most likely due to that theEuprosthenops sp MaSp has the highest polyalanine content among allspecies analyzed so far (Pouchkina-Stantcheva, N. N. & McQueen-Mason, S.J. Molecular studies of a novel dragline silk from a nursery web spider,Euprosthenops sp. (Pisauridae). Comp Biochem Physiol B Biochem Mol Biol138, 371-376 (2004)).

Attempts to produce artificial spider silks have employed natural orsynthetic gene fragments encoding dragline silk proteins, since nofull-length gene has yet been reported. Recombinant dragline silkproteins have been expressed in various systems including bacteria,yeast, mammalian cells, plants, insect cells, transgenic silkworms andtransgenic goats. See e.g. Lewis, R. V. et al. Expression andpurification of a spider silk protein: a new strategy for producingrepetitive proteins. Protein Expr. Purif. 7, 400-406 (1996); Fahnestock,S. R. & Irwin, S. L. Synthetic spider dragline silk proteins and theirproduction in Escherichia coli. Appl. Microbiol. Biotechnol. 47, 23-32(1997); Arcidiacono, S. et al. Purification and characterization ofrecombinant spider silk expressed in Escherichia coli. Appl. Microbiol.Biotechnol. 49, 31-38 (1998); Fahnestock, S. R. & Bedzyk, L. A.Production of synthetic spider dragline silk protein in Pichia pastoris.Appl. Microbiol. Biotechnol. 47, 33-39 (1997); and Lazaris, A. et al.Spider silk fibers spun from soluble recombinant silk produced inmammalian cells. Science 295, 472-476 (2002).

WO 2004/016651 (The University of York) discloses nucleic acid sequencescoding for internal, repetitive parts of MaSp1 proteins fromEuprosthenops sp. No protein is expressed.

Huemmerich, D. et al. Primary structure elements of spider draglinesilks and their contribution to protein solubility. Biochemistry 43,13604-13612 (2004) discloses a synthetic gene, “(AQ)₁₂NR3”, coding forrepetitive Ala-rich and Gly/Gln-rich fragments and a non-repetitivefragment, all derived from ADF3 from Araneus. The gene is expressed intoa soluble protein (59.8 kD, >528 aa), which aggregates but does not formpolymers or fibers. The alanine content of the protein is 10-15%.

WO 03/057727 discloses expression of soluble recombinant silkpolypeptides in mammalian cell lines and animals. One expressed silkpolypeptide (ADF-3; 60 kD, 652 aa) consists of a repetitive unit and anon-repetitive hydrophilic domain. Another expressed silk polypeptide(ADF-3 His; 63 kD, 677 aa) consists of a repetitive unit, anon-repetitive hydrophilic domain, a c-myc epitope and a six-Histidinetag. The repetitive unit has a low content of Ala (10-20%). The obtainedsilk polypeptides exhibit poor solubility in aqueous media and/or formprecipitates. Since the obtained silk polypeptides do not polymerizespontaneously, spinning is required to obtain polymers or fibers.

Several factors complicate the expression of dragline silk proteins. Dueto the highly repetitive nature of the genes, and the concomitantrestricted amino acid composition of the proteins, transcription andtranslation errors occur. Depletion of tRNA-pools in microbialexpression systems, with subsequent discontinuous translation, leadingto premature termination of protein synthesis might be another reason.Other reasons discussed for truncation of protein synthesis aresecondary structure formation of the mRNA, and recombination of thegenes. Native MaSp genes larger than 2.5 kb have been shown to beinstable in bacterial hosts. Additionally, there are difficulties inmaintaining the recombinant silk proteins in soluble form, since bothnatural-derived dragline silk fragments and designed block copolymers,especially MaSp1/ADF-4-derived proteins, easily self-assemble intoamorphous aggregates, causing precipitation and loss of protein. SeeHuemmerich, D. et al. Primary structure elements of spider draglinesilks and their contribution to protein solubility. Biochemistry 43,13604-13612 (2004) and Lazaris, A. et al. Spider silk fibers spun fromsoluble recombinant silk produced in mammalian cells. Science 295,472-476 (2002).

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a novel spider silkprotein, which can provide spider silk fibers.

It is another object of the present invention to provide a water-solublespider silk protein, which can readily be manipulated to self-polymerizeinto fibers at wish. This allows for unique applications, such asculturing of eukaryotic cells on the fibers. Furthermore, this propertyallows for all the following steps to be undertaken under physiologicalconditions, which decreases the risk for toxicity and proteindenaturation.

It is yet another object of the present invention to provide fibers of anovel spider silk protein.

It is one object of the present invention to provide spider silkproteins in large scale, which proteins can readily be manipulated toself-polymerize into fibers at wish.

It is also an object of the invention to provide methods of producingsilk proteins and fibers of spider silk proteins.

For these and other objects that will be evident from the followingdisclosure, the present invention provides according to one aspect anisolated major ampullate spidroin protein, wherein the protein consistsof from 150 to 420 amino acid residues and is defined by the formulaREP-CT, wherein REP is a protein fragment having from 80 to 300 aminoacid residues, wherein said fragment is selected from the group ofL(AG)_(n)L (SEQ ID NO: 17), L(AG)_(n)AL (SEQ ID NO: 18), L(GA)_(n)L (SEQID NO: 19), L(GA)_(n)GL (SEQ ID NO: 20), wherein n is an integer from 4to 8;

each individual A segment is an amino acid sequence of from 8 to 18amino acid residues, wherein from 0 to 3 of the amino acid residues arenot Ala, and the remaining amino acid residues are Ala; each individualG segment is an amino acid sequence of from 12 to 30 amino acidresidues, wherein at least 40% of the amino acid residues are Gly; andeach individual L segment is a linker amino acid sequence of from 0 to20 amino acid residues; and CT is a protein fragment having from 70 to120 amino acid residues, which fragment is a C-terminal fragment derivedfrom a major ampullate spidroin protein, or a derivative thereof.

The present invention is based on the identification of a protein motif,which is sufficient to form silk-like fibers, and the use of said motiffor construction of recombinant MaSp proteins, which are possible toproduce in suitable hosts, such as bacteria, preferably E. coli.

In certain embodiments according to the invention, each individual Asegment has at least 80% identity to an amino acid sequence selectedfrom the group of amino acid residues 7-19, 43-56, 71-83, 107-120,135-147, 171-183, 198-211, 235-248, 266-279, 294-306, 330-342, 357-370,394-406, 421-434, 458-470, 489-502, 517-529, 553-566, 581-594, 618-630,648-661, 676-688, 712-725, 740-752, 776-789, 804-816, 840-853, 868-880,904-917, 932-945, 969-981, 999-1013, 1028-1042 and 1060-1073 of SEQ IDNO: 3; amino acid residues 31-42, 61-75, 90-104, 122-135 and 153-171 ofSEQ ID NO: 9; amino acid residues 12-25, 46-60, 75-88, 112-119, 150-158and 173-180 of SEQ ID NO: 13; amino acid residues 31-42 of SEQ ID NO:14; and amino acid residues 122-135 of SEQ ID NO: 15. In specificembodiments, each individual A segment is an amino acid sequenceselected from this group of amino acid sequences.

In some embodiments according to the invention, each individual Gsegment has at least 80% identity to an amino acid sequence selectedfrom the group of amino acid residues 20-42, 57-70, 84-106, 121-134,148-170, 184-197, 212-234, 249-265, 280-293, 307-329, 343-356, 371-393,407-420, 435-457, 471-488, 503-516, 530-552, 567-580, 595-617, 631-647,662-675, 689-711, 726-739, 753-775, 790-803, 817-839, 854-867, 881-903,918-931, 946-968, 982-998, 1014-1027, 1043-1059 and 1074-1092 of SEQ IDNO: 3; SEQ ID NO: 5; SEQ ID NO: 6; SEQ ID NO: 7; amino acid residues11-30, 43-60, 76-89, 105-121 and 136-152 of SEQ ID NO: 9; and amino acidresidues 1-11, 26-45, 61-74, 89-111, 120-149 and 159-172 of SEQ ID NO:13. In specific embodiments, each individual G segment is identical toan amino acid sequence selected from this group of amino acid sequences.

In certain embodiments according to the invention, said CT fragment hasat least 50% identity to SEQ ID NO: 8 or at least 80% identity to anamino acid sequence selected from the group consisting of SEQ ID NO: 4,amino acid residues 172-269 of SEQ ID NO: 9, amino acid residues 181-276of SEQ ID NO: 13 and amino acid residues 172-269 of SEQ ID NO: 16 aswell as any amino acid sequence of FIG. 3, in particular the MaSp1sequences of FIG. 3. In specific embodiments, said CT fragment is anamino acid sequence selected from this group of amino acid sequences.

In certain embodiments according to the invention, the content oflipopolysaccharides (LPS) and other pyrogens in the isolated majorampullate spidroin protein is 1 endotoxin unit (EU)/mg protein or lower.

According to another aspect, the present invention provides an isolatedfusion protein consisting of a first protein fragment, which is a majorampullate spidroin protein, and a second protein fragment, wherein saidsecond protein fragment comprises a fusion partner and a cleavage agentrecognition site, wherein said first protein fragment is coupled viasaid cleavage agent recognition site to said fusion partner.

The present invention provides an isolated fusion protein selected fromthe group of X-REP-CT, and REP-CT-X, wherein REP and CT are proteinfragments according to the invention; and X is a protein fragmentcomprising a fusion partner and a cleavage agent recognition site;wherein the combined protein fragment REP-CT is coupled via saidcleavage agent recognition site to said fusion partner.

In certain embodiments according to the invention, the content of LPSand other pyrogens in the isolated fusion protein is 1 EU/mg protein orlower.

According to yet another aspect, the present invention provides a methodof producing a major ampullate spidroin protein according to theinvention, comprising the steps of: (i) providing a solution of a fusionprotein according to the invention in a liquid medium, (ii) adding tosaid liquid medium a suitable cleaving agent for achieving cleavage ofthe fusion protein at the cleavage agent recognition site, and therebyobtaining the major ampullate spidroin protein; and optionally (iii)isolating the major ampullate spidroin protein obtained in step (ii)from said liquid medium.

The present invention also provides a method of producing a polymer of amajor ampullate spidroin protein according to the invention, comprisingthe steps of: (i) providing a solution of a fusion protein according tothe invention in a liquid medium, (ii) adding to said liquid medium asuitable cleaving agent for achieving cleavage of the fusion protein atthe cleavage agent recognition site, and thereby obtaining the majorampullate spidroin protein; (iii) allowing the major ampullate spidroinprotein obtained in step (ii) to polymerize in the liquid medium; andoptionally (iv) isolating the polymer obtained in step (iii) from saidliquid medium. In a preferred method, said step (iii) further comprisesproviding an interface between said liquid medium and another phaseselected from the group consisting of a gas phase, a liquid phase and asolid phase, wherein said polymerizing initiates at said interface or ina region surrounding said interface. In a preferred method, said liquidmedium is an aqueous medium and said other phase is selected from thegroup consisting of air and water-immiscible organic solvents.

According to another aspect, the present invention provides an isolatedpolynucleic acid molecule comprising a nucleic acid sequence whichencodes a major ampullate spidroin protein according to the invention,or its complementary nucleic acid sequence.

According to yet another aspect, the present invention provides anisolated polynucleic acid molecule comprising a nucleic acid sequencewhich encodes a fusion protein according to the invention, or itscomplementary nucleic acid sequence.

Another aspect of the invention resides in a method of producing asoluble fusion protein according to the invention, comprising the stepsof: (i) expressing a polynucleic acid molecule encoding a soluble fusionprotein according to the invention in a suitable host; and (ii)isolating the soluble fusion protein obtained in step (i). Optionally,said step (ii) of isolating the soluble fusion protein involves removalof LPS and other pyrogens.

The present invention also provides a method of producing a majorampullate spidroin protein according to the invention, comprising thesteps of: (i) expressing a polynucleic acid molecule encoding a solublefusion protein according to the invention in a suitable host; (ii)isolating the soluble fusion protein obtained in step (i); (iii)providing a solution of said soluble fusion protein obtained in step(ii) in a liquid medium, (iv) adding to said liquid medium a suitablecleaving agent for achieving cleavage of the fusion protein at thecleavage agent recognition site, and thereby obtaining the majorampullate spidroin protein; and optionally (v) isolating the majorampullate spidroin protein obtained in step (iv) from said liquidmedium. Further optionally, said step (ii) of isolating the solublefusion protein, and optionally step (v) of isolating the major ampullatespidroin protein, involve(s) removal of LPS and other pyrogens.

The present invention further provides a method of producing a polymerof a major ampullate spidroin protein according to the invention,comprising the steps of: (i) expressing a polynucleic acid moleculeencoding a soluble fusion protein according to the invention in asuitable host; (ii) isolating the soluble fusion protein obtained instep (i); (iii) providing a solution of said soluble fusion proteinobtained in step (ii) in a liquid medium, (iv) adding to said liquidmedium a suitable cleaving agent for achieving cleavage of the fusionprotein at the cleavage agent recognition site, and thereby obtainingthe major ampullate spidroin protein; (v) allowing the major ampullatespidroin protein obtained in step (iv) to polymerize in the liquidmedium; and optionally (vi) isolating the polymer obtained in step (v)from said liquid medium. In a preferred method, said step (v) furthercomprises providing an interface between said liquid medium and anotherphase selected from the group consisting of a gas phase, a liquid phaseand a solid phase, wherein said polymerizing initiates at said interfaceor in a region surrounding said interface. In a preferred method, saidliquid medium is an aqueous medium and said other phase is selected fromthe group consisting of air and water-immiscible organic solvents.

According to another aspect, the present invention provides a polymer ofa major ampullate spidroin protein according to the invention. Thepresent invention also provides a polymer of a major ampullate spidroinprotein obtainable by a method according to the invention. In apreferred embodiment, said polymer is a fiber. In other preferredembodiments, said polymer forms a structure selected from the groupconsisting of a foam, a gel, a mesh or a film.

According to yet another aspect, the present invention provides a noveluse of a protein fragment comprising a fusion partner and a cleavageagent recognition site for the manufacture of a fusion proteincomprising said protein fragment coupled via said cleavage agentrecognition site to a spider silk protein fragment. In preferredembodiments, said spider silk protein fragment consists of from 150 to420 amino acid residues.

According to a final aspect, the present invention provides an isolatedpolynucleic acid molecule comprising a nucleic acid sequence selectedfrom the group consisting of SEQ ID NO: 1 and nucleic acid sequencesencoding SEQ ID NOS: 2-16, or its complementary nucleic acid sequences.The present invention also provides use of the isolated polynucleic acidmolecule for the manufacture of a non-natural gene encoding a spidersilk protein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an alignment of the segments within the repetitive part ofEuprosthenops australis MaSp1 protein, i.e. SEQ ID NO: 3.

FIG. 2A illustrates a schematic, predicted structural organization ofthe repetitive part of Euprosthenops australis MaSp1 protein (SEQ ID NO:3). The various peptide segments shown correspond to SEQ ID NOS: 21-23and 5-6, respectively.

FIG. 2B illustrates schematic, predicted structural organizations of thespidroin proteins constructed according to examples 5-8 (SEQ ID NOS:9-13).

FIG. 3 is an alignment of C-terminal regions of MaSp1 and MaSp2,illustrating their conserved nature (SEQ ID NOS: 24-55).

FIG. 4 illustrates macroscopic appearances of fibers formed fromspidroin proteins constructed according to examples 5-8. (A):6Gly/Ala-CT_(hyb) protein (SEQ ID NO: 13) fibers, bar 0.5 cm; (B):5Gly/Ala-CT_(nat) (SEQ ID NO: 9) protein fibers, bar 1 cm. (C):5Gly/Ala-CT_(nat) (SEQ ID NO: 9) protein fibers, bar 1 cm.

FIG. 5 shows scanning electron microscopy (SEM) micrographs of fibersformed from spidroin proteins constructed according to examples 5-8.Single fibers (a) and gel-phase (b, c) from 6Gly/Ala-CT_(hyb) (SEQ IDNO: 13). Fibers of 5Gly/Ala-CT_(nat) (SEQ ID NO: 9), drawn in 75%methanol, air-dried and applied on SEM-stubs (d, e, f). Fiber twistedbefore air-drying (e), end of fiber (f).

FIG. 6 displays a circular dichroism (CD) spectrum of 6Gly/Ala-CT_(hyb)(SEQ ID NO: 13) fiber.

FIG. 7 illustrates the results from a mouse mast cell toxicity study,showing the numbers of live and dead cells after three days of culturein the presence or absence of in vitro produced silk fibers.

FIG. 8 is a picture of HEK293 cells following exposure to in vitroproduced silk fibers in a biocompatibility study.

FIG. 9 is a stress-strain curve displaying the tensile strength ofdouble drawn fibers from 5Gly/Ala-CT_(nat) (SEQ ID NO: 9).

FIG. 10 shows SEM micrographs of recombinant fibers from5Gly/Ala-CT_(nat) (SEQ ID NO: 9). a,b, Spontaneously formed fibers. Theclose-up image (b) shows the fibrillar substructure. The small fibrilthat bulges out (arrow) has a width of about 300 nm. c-f, Fibers aftertwo stretching-relaxation cycles. c and d shows the same fiber atdifferent magnifications. e shows a cut fiber end, and f shows a pointof breakage after tensile testing.

DETAILED DISCLOSURE OF THE INVENTION

The present invention is generally based on the identification of aspidroin protein motif, which is sufficient for recombinant productionof spider silk fibers. The motif is based on the deduced amino acidsequence from cloning and sequencing of a partial major spidroin 1(MaSp1) cDNA from Euprosthenops australis. It follows that the isolatedMaSp1 cDNA is useful as a starting point for construction of novelspidroin genes, such as those reported herein. The polymers which areformed from the proteins resulting from the novel spidroin cDNAs areuseful for their physical properties, especially the useful combinationof high strength, elasticity and light weight. They are also useful fortheir ability to support cell adherence and growth. The properties ofdragline silk are attractive in development of new materials for medicalor technical purposes. In particular, spider silks according to theinvention are useful in medical devices, such as implants and medicalproducts, such as wound closure systems, band-aids, sutures, wounddressings, and scaffolds for tissue engineering and guided cellregeneration. Spider silks according to the invention are alsoparticularly useful for use as textile or fabric, such as in parachutes,bulletproof clothing, seat belts, etc.

The term “fiber” as used herein relates to polymers having a thicknessof at least 1 μm, preferably macroscopic polymers that are visible tothe human eye, i.e. having a thickness of at least 1 μm, and have aconsiderable extension in length compared to its thickness, preferablyabove 5 mm. The term “fiber” does not encompass unstructured aggregatesor precipitates.

The terms “major ampullate spidroin proteins”, “spidroin proteins” areused interchangeably throughout the description and encompass all knownmajor ampullate spidroin proteins, typically abbreviated “MaSp”, or“ADF” in the case of Araneus diadematus. These major ampullate spidroinproteins are generally of two types, 1 and 2. These terms furthermoreinclude the new proteins according to the invention, as defined in theappended claims, and other non-natural proteins with a high degree ofidentity and/or similarity to the known major ampullate spidroinproteins.

The present inventors have utilized the identified spidroin proteinmotif for construction of novel gene constructs, coding for non-naturalspidroin proteins. It has been found that a major ampullate spidroinprotein consisting of from 150 to 420 amino acid residues, i.e. morethan or equal to 150, preferably more than or equal to 220, preferablymore than or equal to 250, and less than or equal to 420, preferablyless than or equal to 380 amino acid residues, preferably less than orequal to 320 amino acid residues, preferably less than or equal to 280amino acid residues, such as 220-360 amino acid residues, can berecombinantly produced, e.g. in bacteria or other suitable productionorganisms. The resulting spidroin proteins spontaneously formmacroscopic silk fibers according to the invention. This is a surprisingresult, since the naturally occurring spidroin proteins and previouslyknown, recombinantly produced, fiber-forming spidroin proteins areconsiderably longer than the proteins according to the invention.Moreover, the naturally occurring spidroin proteins and previouslyknown, recombinantly produced, fiber-forming spidroin proteins tend tocontain a large number of internal repeats and require use of spinningand/or harsh solvents for polymerization.

It is here for the first time shown that spidroin proteins canspontaneously form fibers in vitro. The data presented herein also showthat only a fraction of the spidroin sequence need to be present todictate fiber formation. Moreover, a species hybrid containing aEuprosthenops repetitive domain and a Nephila non-repetitive C-terminaldomain (c.f. Example 6C) forms fibers as well, indicating that thefiber-forming potential of this motif is robust.

In its general aspect, the major ampullate spidroin protein according tothe invention is defined by the formula REP-CT. The REP protein fragmentand the CT protein fragment are covalently coupled, typically via apeptide bond.

The protein fragment REP has a repetitive character, alternating betweenalanine-rich stretches and glycine-rich stretches. The REP fragmentgenerally contains more than 80, such as more than 140, and less than300, preferably less than 240, such as less than 200, amino acidresidues, and can itself be divided into several L (linker) segments, A(alanine-rich) segments and G (glycine-rich) segments, as will beexplained in more detail below. Typically, said linker segments, whichare optional, are located at the REP fragment terminals, while theremaining segments are in turn alanine-rich and glycine-rich. Thus, theREP fragment can generally have either of the following structures,wherein n is an integer:

-   L(AG)_(n)L (SEQ ID NO: 17), such as LA₁G₁A₂G₂A₃G₃A₄G₄A₅G₅L (SEQ ID    NO: 56);-   L(AG)_(n)AL (SEQ ID NO: 18), such as LA₁G₁A₂G₂A₃G₃A₄G₄A₅G₅A₆L (SEQ    ID NO: 57);-   L(GA)_(n)L (SEQ ID NO: 19), such as LG₁A₁G₂A₂G₃A₃G₄A₄G₅A₅L (SEQ ID    NO: 58); or-   L(GA)_(n)GL (SEQ ID NO: 20), such as LG₁A₁G₂A₂G₃A₃G₄A₄G₅A₅G₆L (SEQ    ID NO: 59).    It follows that it is not critical whether an alanine-rich or a    glycine-rich segment is adjacent to the N-terminal or C-terminal    linker segments. It is preferred that n is an integer from 4 to 8,    more preferred from 4 to 6, i.e. n=4, n=5 or n=6.

In preferred embodiments, the alanine content of the REP fragmentaccording to the invention is above 20%, preferably above 25%, morepreferably above 30%, and below 50%, preferably below 40%, morepreferably below 35%. This is advantageous, since it is contemplatedthat a higher alanine content provides a stiffer and/or stronger and/orless extendible fiber. The reason for this is likely to be that a higheralanine content is associated with a higher content of β-sheetstructures in the fiber. Thus, in a preferred embodiment, the β-sheetcontent in a polymer, such as a fiber, of the major ampullate spidroinprotein according to the invention is above 50%, i.e. more than 50% ofthe secondary structure of the protein is in β-sheet form.

In certain embodiments, the REP fragment is void of proline residues,i.e. there are no Pro residues in the REP fragment.

Now turning to the segments that constitute the REP fragment accordingto the invention, it shall be emphasized that each segment isindividual, i.e. any two A segments, any two G segments or any two Lsegments of a specific REP fragment may be identical or may not beidentical. Thus, it is not a general feature of the invention that eachtype of segment is identical within a specific REP fragment. Rather, thefollowing disclosure provides the skilled person with guidelines how todesign individual segments and gather them into a REP fragment, which isa part of a functional spidroin protein according to the invention.

It has been concluded from experimental data presented herein that eachindividual A segment is an amino acid sequence having from 8 to 18 aminoacid residues. It is preferred that each individual A segment containsfrom 13 to 15 amino acid residues. It is also possible that a majority,or more than two, of the A segments contain from 13 to 15 amino acidresidues, and that a minority, such as one or two, of the A segmentscontain from 8 to 18 amino acid residues, such as 8-12 or 16-18 aminoacid residues. A vast majority of these amino acid residues are alanineresidues. More specifically, from 0 to 3 of the amino acid residues arenot alanine residues, and the remaining amino acid residues are alanineresidues. Thus, all amino acid residues in each individual A segment arealanine residues, with no exception or the exception of one, two orthree amino acid residues, which can be any amino acid. It is preferredthat the alanine-replacing amino acid(s) is (are) natural amino acids,preferably individually selected from the group of serine, glutamicacid, cysteine and glycine, more preferably serine. Of course, it ispossible that one or more of the A segments are all-alanine segments,while the remaining A segments contain 1-3 non-alanine residues, such asserine, glutamic acid, cysteine or glycine.

In a preferred embodiment, each A segment contains 13-15 amino acidresidues, including 10-15 alanine residues and 0-3 non-alanine residuesas described above. In a more preferred embodiment, each A segmentcontains 13-15 amino acid residues, including 12-15 alanine residues and0-1 non-alanine residues as described above.

It is preferred that each individual A segment has at least 80% identityto an amino acid sequence selected from the group of amino acid residues7-19, 43-56, 71-83, 107-120, 135-147, 171-183, 198-211, 235-248,266-279, 294-306, 330-342, 357-370, 394-406, 421-434, 458-470, 489-502,517-529, 553-566, 581-594, 618-630, 648-661, 676-688, 712-725, 740-752,776-789, 804-816, 840-853, 868-880, 904-917, 932-945, 969-981, 999-1013,1028-1042 and 1060-1073 of SEQ ID NO: 3. Each sequence of this groupcorresponds to a segment of the naturally occurring sequence ofEuprosthenops australis MaSp1 protein, which is deduced from cloning ofthe corresponding cDNA, see Examples 1-2 and FIG. 1-2A. Alternatively,each individual A segment has at least 80% identity to an amino acidsequence selected from the group of amino acid residues 31-42, 61-75,90-104, 122-135 and 153-171 of SEQ ID NO: 9, amino acid residues 12-25,46-60, 75-88, 112-119, 150-158 and 173-180 of SEQ ID NO: 13, amino acidresidues 31-42 of SEQ ID NO: 14 and amino acid residues 122-135 of SEQID NO: 15. Each sequence of this group corresponds to a segment ofexpressed, non-natural spidroin proteins according to the invention,which proteins have capacity to form silk fibers under appropriateconditions. See Examples 5-8, 12 and FIG. 2B. Without wishing to bebound by any particular theory, it is envisaged that A segmentsaccording to the invention form helical structures or beta sheets.

The term “% identity”, as used throughout the specification and theappended claims, is calculated as follows. The query sequence is alignedto the target sequence using the CLUSTAL W algorithm (Thompson, J. D.,Higgins, D. G. and Gibson, T. J., Nucleic Acids Research, 22: 4673-4680(1994)). The amino acid residues at each position are compared, and thepercentage of positions in the query sequence that have identicalcorrespondences in the target sequence is reported as % identity.

The term “% similarity”, as used throughout the specification and theappended claims, is calculated as described for “% identity”, with theexception that the hydrophobic residues Ala, Val, Phe, Pro, Leu, Ile,Trp, Met and Cys are similar; the basic residues Lys, Arg and His aresimilar; the acidic residues Glu and Asp are similar; and thehydrophilic, uncharged residues Gln, Asn, Ser, Thr and Tyr are similar.The remaining natural amino acid Gly is not similar to any other aminoacid in this context.

Throughout this description, alternative embodiments according to theinvention fulfill, instead of the specified percentage of identity, thecorresponding percentage of similarity. Other alternative embodimentsfulfill the specified percentage of identity as well as another, higherpercentage of similarity, selected from the group of preferredpercentages of identity for each sequence. For example, a sequence maybe 70% similar to another sequence; or it may be 70% identical toanother sequence; or it may be 70% identical and 90% similar to anothersequence.

In preferred embodiments according to the invention, each individual Asegment has at least 90%, more preferably 95%, most preferably 100%,identity to an amino acid sequence selected from the group of amino acidresidues 7-19, 43-56, 71-83, 107-120, 135-147, 171-183, 198-211,235-248, 266-279, 294-306, 330-342, 357-370, 394-406, 421-434, 458-470,489-502, 517-529, 553-566, 581-594, 618-630, 648-661, 676-688, 712-725,740-752, 776-789, 804-816, 840-853, 868-880, 904-917, 932-945, 969-981,999-1013, 1028-1042 and 1060-1073 of SEQ ID NO: 3; amino acid residues31-42, 61-75, 90-104, 122-135 and 153-171 of SEQ ID NO: 9; amino acidresidues 12-25, 46-60, 75-88, 112-119, 150-158 and 173-180 of SEQ ID NO:13; amino acid residues 31-42 of SEQ ID NO: 14; and amino acid residues122-135 of SEQ ID NO: 15. Thus, in certain embodiments according to theinvention, each individual A segment is identical to an amino acidsequence selected from the above-mentioned amino acid segments.

Furthermore, it has been concluded from experimental data presentedherein that each individual G segment is an amino acid sequence of from12 to 30 amino acid residues. It is preferred that each individual Gsegment consists of from 14 to 23 amino acid residues. At least 40% ofthe amino acid residues of each G segment are glycine residues.Typically the glycine content of each individual G segment is in therange of 40-60%.

It is preferred that each individual G segment has at least 80% identityto an amino acid sequence selected from the group of amino acid residues20-42, 57-70, 84-106, 121-134, 148-170, 184-197, 212-234, 249-265,280-293, 307-329, 343-356, 371-393, 407-420, 435-457, 471-488, 503-516,530-552, 567-580, 595-617, 631-647, 662-675, 689-711, 726-739, 753-775,790-803, 817-839, 854-867, 881-903, 918-931, 946-968, 982-998,1014-1027, 1043-1059 and 1074-1092 of SEQ ID NO: 3. Each sequence ofthis group corresponds to a segment of the naturally occurring sequenceof Euprosthenops australis MaSp1 protein, which is deduced from cloningof the corresponding cDNA, see Examples 1-2 and FIG. 1-2A.Alternatively, each individual G segment has at least 80% identity to anamino acid sequence selected from the group of amino acid residues11-30, 43-60, 76-89, 105-121 and 136-152 of SEQ ID NO: 9 and amino acidresidues 1-11, 26-45, 61-74, 89-111, 120-149 and 159-172 of SEQ ID NO:13. Each sequence of this group corresponds to a segment of expressed,non-natural spidroin proteins according to the invention, which proteinshave capacity to form silk fibers under appropriate conditions. SeeExamples 5-8, 12 and FIG. 2B.

In preferred embodiments according to the invention, each individual Gsegment has at least 90%, more preferably 95%, most preferably 100%,identity to an amino acid sequence selected from the group of amino acidresidues 20-42, 57-70, 84-106, 121-134, 148-170, 184-197, 212-234,249-265, 280-293, 307-329, 343-356, 371-393, 407-420, 435-457, 471-488,503-516, 530-552, 567-580, 595-617, 631-647, 662-675, 689-711, 726-739,753-775, 790-803, 817-839, 854-867, 881-903, 918-931, 946-968, 982-998,1014-1027, 1043-1059 and 1074-1092 of SEQ ID NO: 3; amino acid residues11-30, 43-60, 76-89, 105-121 and 136-152 of SEQ ID NO: 9; and amino acidresidues 1-11, 26-45, 61-74, 89-111, 120-149 and 159-172 of SEQ ID NO:13. Thus, in certain embodiments according to the invention, eachindividual G segment is identical to an amino acid sequence selectedfrom the above-mentioned amino acid segments.

In certain embodiments, the first two amino acid residues of each Gsegment according to the invention are not -Gln-Gln-.

In certain embodiments, the position corresponding to the conserved Tyrresidue (i.e. corresponding to position 16 in SEQ ID NO: 5, position 10in SEQ ID NO: 6 and position 7 in SEQ ID NO: 7) is not Phe in any Gsegment according to the invention.

In certain embodiments, the position corresponding to the conserved Tyrresidue (i.e. corresponding to position 16 in SEQ ID NO: 5, position 10in SEQ ID NO: 6 and position 7 in SEQ ID NO: 7) is Tyr in each G segmentaccording to the invention.

It follows that certain embodiments of the proteins according to theinvention display a combination of the above-mentioned limitations, i.e.the first two amino acid residues of each G segment according to theinvention are not -Gln-Gln-, and the conserved Tyr residue (i.e.corresponding to position 16 in SEQ ID NO: 5, position 10 in SEQ ID NO:6 and position 7 in SEQ ID NO: 7) is Tyr in each G segment according tothe invention. In certain embodiments, the above-mentioned limitations,taken separately or in any possible combination, can be further combinedwith the limitation that the REP fragment is void of proline residues,as discussed hereinabove.

With reference to FIGS. 1-2 and Examples 3-4, there are the threesubtypes of the G segment according to the invention. Thisclassification is based upon careful analysis of the Euprosthenopsaustralis MaSp1 protein sequence (FIGS. 1-2A), and the information hasbeen employed and verified in the construction of novel, non-naturalspider silk proteins (FIG. 2B).

The first subtype of the G segment according to the invention isrepresented by the amino acid one letter consensus sequenceGQG(G/S)QGG(Q/Y)GG (L/Q)GQGGYGQGA GSS, as shown in FIG. 2A and SEQ IDNO: 5. This first, and generally the longest, G segment subtypetypically contains 23 amino acid residues, but may contain as little as17 amino acid residues, and lacks charged residues or contain onecharged residue. Thus, it is preferred that this first G segment subtypecontains 17-23 amino acid residues, but it is contemplated that it maycontain as few as 12 or as many as 30 amino acid residues. Withoutwishing to be bound by any particular theory, it is envisaged that thissubtype forms coil structures or 3₁-helix structures. Representative Gsegments of this first subtype are amino acid residues 20-42, 84-106,148-170, 212-234, 307-329, 371-393, 435-457, 530-552, 595-617, 689-711,753-775, 817-839, 881-903, 946-968, 1043-1059 and 1074-1092 of SEQ IDNO: 3; amino acid residues 11-30, 105-121 and 136-152 of SEQ ID NO: 9;and amino acid residues 26-45 and 89-111 of SEQ ID NO: 13. Alternative Gsegments of this first subtype are amino acid residues 120-149 and159-172 of SEQ ID NO: 13. In certain embodiments, the first two aminoacid residues of each G segment of this first subtype according to theinvention are not -Gln-Gln-.

The second subtype of the G segment according to the invention isrepresented by the amino acid one letter consensus sequenceGQGGQGQG(G/R)Y GQG(A/S)G(S/G)S, as shown in FIG. 2A and SEQ ID NO: 6.This second, generally mid-sized, G segment subtype typically contains17 amino acid residues and lacks charged residues or contain one chargedresidue. It is preferred that this second G segment subtype contains14-20 amino acid residues, but it is contemplated that it may contain asfew as 12 or as many as 30 amino acid residues. Without wishing to bebound by any particular theory, it is envisaged that this subtype formscoil structures. Representative G segments of this second subtype areamino acid residues 249-265, 471-488, 631-647 and 982-998 of SEQ ID NO:3; and amino acid residues 43-60 of SEQ ID NO: 9.

The third subtype of the G segment according to the invention isrepresented by the amino acid one letter consensus sequenceG(R/Q)GQG(G/R)YGQG (A/S/V)GGN, as shown in FIG. 2A and SEQ ID NO: 7.This third G segment subtype typically contains 14 amino acid residues,and is generally the shortest of the G segment subtypes according to theinvention. It is preferred that this third G segment subtype contains12-17 amino acid residues, but it is contemplated that it may contain asmany as 23 amino acid residues. Without wishing to be bound by anyparticular theory, it is envisaged that this subtype forms turnstructures. Representative G segments of this third subtype are aminoacid residues 57-70, 121-134, 184-197, 280-293, 343-356, 407-420,503-516, 567-580, 662-675, 726-739, 790-803, 854-867, 918-931, 1014-1027of SEQ ID NO: 3; amino acid residues 76-89 of SEQ ID NO: 9; and aminoacid residues 61-74 of SEQ ID NO: 13. An alternative G segment of thisthird subtype is amino acid residues 1-11 of SEQ ID NO: 13.

Thus, in preferred embodiments, each individual G segment has at least80%, preferably 90%, more preferably 95%, identity to an amino acidsequence selected from SEQ ID NO: 5, SEQ ID NO: 6 and SEQ ID NO: 7.

In a preferred embodiment of the alternating sequence of A and Gsegments of the REP fragment, every second G segment is of the firstsubtype, while the remaining G segments are of the third subtype, e.g. .. . A₁G_(short)A₂G_(long)A₃G_(short)A₄G_(long)A₅G_(short) . . . (SEQ IDNO: 56) In another preferred embodiment of the REP fragment, one Gsegment of the second subtype interrupts the G segment regularity via aninsertion, e.g. . . .A₁G_(short)A₂G_(long)A₃G_(mid)A₄G_(short)A₅G_(long) . . . (SEQ ID NO:56)

Each individual L segment represents an optional linker amino acidsequence, which may contain from 0 to 20 amino acid residues, such asfrom 0 to 10 amino acid residues. While this segment is optional and notfunctionally critical for the spidroin protein, its presence stillallows for fully functional spidroin proteins, forming spider silkfibers according to the invention. There are also linker amino acidsequences present in the repetitive part (SEQ ID NO: 3) of the deducedamino acid sequence of the MaSp1 protein from Euprosthenops australis.In particular, the amino acid sequence of a linker segment may resembleany of the described A or G segments, but usually not sufficiently tomeet their criteria as defined herein.

As shown in FIG. 2A, a linker segment arranged at the C-terminal part ofthe REP fragment can be represented by the amino acid one letterconsensus sequences ASASAAASAA STVANSVS (SEQ ID NO: 60) and ASAASAAA(SEQ ID NO: 61), which are rich in alanine. In fact, the second sequencecan be considered to be an A segment according to the invention, whilethe first sequence has a high degree of similarity to A segmentsaccording to the invention. Another example of a linker segmentaccording the invention has the one letter amino acid sequence GSAMGQGS(SEQ ID NO: 62), which is rich in glycine and has a high degree ofsimilarity to G segments according to the invention.

Representative L segments are amino acid residues 1-6 and 1093-1110 ofSEQ ID NO: 3; amino acid residues 1-10 and 153-171 of SEQ ID NO: 9; andamino acid residues 173-180 of SEQ ID NO: 13, but the skilled person inthe art will readily recognize that there are many suitable alternativeamino acid sequences for these segments. In one embodiment of the REPfragment according to the invention, one of the L segments contains 0amino acids, i.e. one of the L segments is void. In another embodimentof the REP fragment according to the invention, both L segments contain0 amino acids, i.e. both L segments are void. Thus, these embodiments ofthe REP fragments according to the invention may be schematicallyrepresented as follows: (AG)_(n)L, (AG)_(n)AL, (GA)_(n)L, (GA)_(n)GL;L(AG)_(n), L(AG)_(n)A, L(GA)_(n), L(GA)_(n)G; and (AG)_(n), (AG)_(n)A,(GA)_(n), (GA)_(n)G. Any of these REP fragments are suitable for usewith any CT fragment as defined below.

The C-terminal (CT) fragment of the spidroin protein according to theinvention has a high degree of similarity to the C-terminal amino acidsequence of spidroin proteins. As shown in FIG. 3, this amino acidsequence is well conserved among various species and spidroin proteins,including MaSp1 and MaSp2. It is demonstrated in the following examplesthat it is not critical which specific CT fragment is present inspidroin proteins according to the invention, as long as the CT fragmentis not entirely missing. Thus, the CT fragment according to theinvention can be selected from any of the amino acid sequences shown inFIG. 3 or sequences with a high degree of similarity. It is notable thatthe CT_(hyb) fragment of SEQ ID NO: 13 has 96% identity to the consensusamino acid sequence SEQ ID NO: 8, while the CT_(nat) fragment of SEQ IDNO: 9 displays only 59% identity to the consensus amino acid sequenceSEQ ID NO: 8. This illustrates that a wide variety of C-terminalsequences can be used in the spidroin protein according to theinvention.

The sequence of the CT fragment according to the invention has at least50% identity, preferably at least 60% identity, to the consensus aminoacid sequence SEQ ID NO: 8, which is based on the amino acid sequencesof FIG. 3. In a preferred embodiment, the sequence of the CT fragmentaccording to the invention has at least 65% identity, preferably atleast 70% identity, to amino acid residues 1-71 of the consensus aminoacid sequence SEQ ID NO: 8. In preferred embodiments, the CT fragmentaccording to the invention has furthermore 70%, preferably 80%,similarity to the consensus amino acid sequence SEQ ID NO: 8, or aminoacid residues 1-71 thereof.

Representative CT fragments according to the invention are theEuprosthenops australis sequence SEQ ID NO: 4, the Euprosthenopsaustralis-derived amino acid residues 172-269 of SEQ ID NO: 9 and aminoacid residues 181-276 of SEQ ID NO: 13, alleged to be derived fromEuprosthenops sp (Pouchkina-Stantcheva, N. N. & McQueen-Mason, S. J.Molecular studies of a novel dragline silk from a nursery web spider,Euprosthenops sp. (Pisauridae). Comp Biochem Physiol B Biochem Mol Biol138, 371-376 (2004)), but with a high degree of similarity to MaSp1 fromNephila clavipes and Nephila senegalensis. Thus, according to apreferred aspect of the invention, the CT fragment has at least 80%identity to SEQ ID NO: 4, amino acid residues 172-269 of SEQ ID NO: 9,amino acid residues 181-276 of SEQ ID NO: 13, amino acid residues172-269 of SEQ ID NO: 16 or any individual MaSp1/ADF-4 amino acidsequence of FIG. 3 and Example 4. In preferred aspects of the invention,the CT fragment has at least 90%, such as at least 95% identity, to SEQID NO: 4, amino acid residues 172-269 of SEQ ID NO: 9, amino acidresidues 181-276 of SEQ ID NO: 13, amino acid residues 172-269 of SEQ IDNO: 16 or any individual MaSp1/ADF-4 amino acid sequence of FIG. 3 andExample 4. In preferred aspects of the invention, the CT fragment isidentical to SEQ ID NO: 4, amino acid residues 172-269 of SEQ ID NO: 9,amino acid residues 181-276 of SEQ ID NO: 13, amino acid residues172-269 of SEQ ID NO: 16 or any individual MaSp1/ADF-4 amino acidsequence of FIG. 3 and Example 4.

The CT fragment typically consists of from 70 to 120 amino acidresidues. It is preferred that the CT fragment contains at least 70, ormore than 80, preferably more than 90, amino acid residues. It is alsopreferred that the CT fragment contains at most 120, or less than 110amino acid residues. A typical CT fragment contains approximately 100amino acid residues.

According to another aspect, the present invention provides an isolatedfusion protein consisting of a first protein fragment, which is a majorampullate spidroin protein, preferably consisting of from 150 to 420amino acid residues, and a second protein fragment, which comprises afusion partner and a cleavage agent recognition site. The first proteinfragment is coupled via the cleavage agent recognition site to thefusion partner, i.e. the fusion partner can be cleaved off by treatingthe fusion protein with a suitable cleaving agent under appropriateconditions, providing a major ampullate spidroin protein, preferablyconsisting of from 150 to 420 amino acid residues. An advantage withthis fusion protein is that large amounts thereof can be produced insolution, preferably in a physiological medium, typically a bufferedaqueous medium, such as a 10-100 mM Tris-HCl buffer, pH 6-9, withoutcausing precipitation and other production problems when produced insuitable hosts, such as bacteria, preferably E. coli. The fusionproteins in the solution are soluble for long time periods, typicallydays or weeks, which facilitates large-scale production and decreasesthe risk for protein aggregation. By the terms “soluble” and “insolution” is meant that the protein is not visibly aggregated and doesnot precipitate from the solvent at 60 000×g. At wish, the fusionproteins in the solution can be subjected to cleavage using a suitablecleaving agent, providing a major ampullate spidroin protein whichspontaneously forms silk fibers.

In a preferred aspect, the present invention provides an isolated fusionprotein selected from the group of X-REP-CT and REP-CT-X, preferablyX-REP-CT. REP and CT are protein fragments according to the invention,implying that the resulting MaSp protein of the form REP-CT is a MaSpprotein according to the invention. X is a protein fragment comprising afusion partner and a cleavage agent recognition site as defined above.The combined protein fragment REP-CT is coupled via the cleavage agentrecognition site to the fusion partner.

Fusion partners according to the invention include any protein fragmentwhich improves the solubility and/or stability of its partner proteinfragment, here the MaSp protein according to the invention. The fusionpartner also provides a suitable handle for affinity purification.Without being limited thereto, examples of fusion partners according tothe invention include thioredoxin, maltose-binding protein, glutathioneS-transferase (GST), MTB32-C, Gb1, ZZ and Nus A. The skilled person iswell aware of alternative suitable fusion partners. In a preferredembodiment of the invention, the fusion partner is a thioredoxin moiety(ThrX) in combination with a His tag and an S tag. In one preferredembodiment of the invention, the fusion partner is a ThrX moiety incombination with two His tags, i.e. His-tag/ThrX/His-tag. In anotherpreferred embodiment of the invention, the fusion partner is athioredoxin moiety (ThrX).

The cleavage agent recognition site is situated at that X proteinfragment terminal which is coupled to the MaSp protein fragment, so thatcleavage at the recognition site results in a MaSp protein and a fusionpartner. Without being limited thereto, examples of the cleavage agentrecognition site according to the invention include a thrombinrecognition site having the amino acid sequence LVPRGS (SEQ ID NO: 63)(cleaves between R and G); an enterokinase recognition site having theamino acid sequence DDDK (SEQ ID NO: 64) (cleaves after K); anhydroxylamine recognition site having the amino acid sequence NG(cleaves between N and G); a HRV 3C protease recognition site having theamino acid sequence LGVLFQGP (SEQ ID NO: 65)(cleaves between Q and G); aFactor Xa recognition site having the amino acid sequence I(E/D)GR (SEQID NO: 66) (cleaves after R); a TEV recognition site having the aminoacid sequence EXXYXQ(G/S) (SEQ ID NO: 67), commonly ENLYFQG (SEQ ID NO:68)(cleaves between Q and G/S), an Ac-TEV recognition site having theamino acid sequence EDNLYFQG (SEQ ID NO: 69)(cleaves between Q and G);and a PreScission recognition site having the amino acid sequenceLEVLFQGP (SEQ ID NO: 70) (cleaves between Q and G). Other suitablerecognition sites are the cleavage sites for trypsin, endoproteinase, V8protease, pepsin and CNBr. Further examples of suitable cleavagerecognition sites are well within the reach of the skilled person. In apreferred embodiment of the invention, the cleavage agent recognitionsite is a thrombin recognition site.

In a preferred embodiment, the X fragment according to the invention hasthe structure ThrX/His-tag/S-tag/thrombin cleavage site, and the Xfragment is coupled to the N-terminal of the REP-CT protein fragmentaccording to the invention.

In one preferred embodiment, the X fragment according to the inventionhas the structure His-tag/ThrX/His-tag/thrombin cleavage site, and the Xfragment is coupled to the N-terminal of the REP-CT protein fragmentaccording to the invention.

According to another aspect, the present invention provides a method ofproducing a major ampullate spidroin protein according to the invention.In the first step, a solution of a fusion protein according to theinvention in a liquid medium is provided. Preferably, the fusion proteindoes not aggregate, and therefore, resolubilization procedures are notrequired. The fusion protein can be recombinantly produced and purifiedusing an affinity handle at the fusion protein, such as a His-tag or anysuitable epitope in the fusion protein. The liquid medium can be anysuitable medium, preferably a physiological medium, typically a bufferedaqueous medium, such as a 10-100 mM Tris-HCl buffer, pH 6-9. In thesecond step, a cleavage agent according to the invention is added to theliquid medium in order to achieve cleavage of the fusion protein at thecleavage agent recognition site. As disclosed above, the major ampullatespidroin protein according to the invention is thereby obtained. In athird, optional step, the thus obtained major ampullate spidroin proteinis isolated from the liquid medium using suitable isolation techniques,such as chromatography and/or filtration.

According to yet another aspect, the present invention provides a methodof producing a polymer of a major ampullate spidroin protein accordingto the invention. In the first step, a solution of a fusion proteinaccording to the invention in a liquid medium is provided. Preferably,the fusion protein does not aggregate, and therefore, resolubilizationprocedures are not required. The fusion protein can be recombinantlyproduced and purified using an affinity handle at the fusion protein,such as a His-tag or any suitable epitope in the fusion protein. Theliquid medium can be any suitable medium, preferably a physiologicalmedium, typically a buffered aqueous medium, such as a 10-100 mMTris-HCl buffer, pH 6-9. In the second step, a cleavage agent accordingto the invention is added to the liquid medium in order to achievecleavage of the fusion protein at the cleavage agent recognition site.As disclosed above, the major ampullate spidroin protein according tothe invention is thereby obtained. In the third step, the thus obtainedmajor ampullate spidroin protein is allowed to polymerize in the liquidmedium. The polymerization typically initiates at the interface betweentwo different phases, such as liquid/air, liquid/solid, and water/oilinterfaces. Thus, this third step may also further comprise providing aninterface between the liquid medium and another phase. The other phaseis selected from the group consisting of a gas phase, a liquid phase anda solid phase. As detailed above, the liquid medium is typically anaqueous medium, and suitable other phases are for instance air andwater-immiscible organic solvents, such as oil, e.g. mineral oilsuitable for PCR reactions. The presence of the resulting interfacestimulates polymerization at the interface or in the region surroundingthe interface, which region extends into the liquid medium, such thatsaid polymerizing initiates at said interface or in said interfaceregion. Preferred interfaces include water/air and water/oil interfaces.Polymerization typically occurs spontaneously within minutes or a fewhours, such as within from 1 min to 5 h, upon incubation at roomtemperature. In a fourth, optional step, the thus obtained polymer ofthe major ampullate spidroin protein is isolated from the liquid mediumusing suitable isolation techniques.

As discussed above, fiber formation is induced by proteolytic release ofthe miniature spidroin from the fusion protein. If the cleavage reactionis performed in a tube that is gently wagged from side to side, a fiberis formed at the air-water interface along the tube. The tube can bemade of any suitable material, such as plastic or glass. If the cleavagemixture is allowed to stand still, a film is formed at the air-waterinterface. If oil is added on top of the aqueous cleavage mixture, afilm is formed at the oil-water interface, either if allowed to standstill or if wagged. If the cleavage mixture is foamed, e.g. by bubblingof air or whipping, the foam is stable and solidifies if allowed to dry.

Using the method(s) of the present invention, it is possible torecombinantly produce large amounts of fusion proteins according theinvention, which can be cleaved and allowed to polymerize at desire.This provides a better control of the polymerization process and allowsfor optimization of parameters for obtaining silk fibers with desirableproperties.

The major ampullate spidroin protein according to the invention istypically recombinantly produced using a variety of suitable hosts.According to another aspect, the present invention therefore provides anisolated polynucleic acid molecule comprising a nucleic acid sequencewhich encodes a major ampullate spidroin protein according to theinvention, or its complementary nucleic acid sequence, such as SEQ IDNOS: 9-13, preferably SEQ ID NOS: 9, 12 and 13. These polynucleic acidmolecules as well as polynucleic acid molecules coding for the variousproteins disclosed herein (SEQ ID NOS: 2-16) may also be useful infurther developments of non-natural spidroin proteins or productionsystems therefor.

The fusion protein according to the invention is typically recombinantlyproduced using a variety of suitable hosts, such as bacteria, yeast,mammalian cells, plants, insect cells, and transgenic animals. It ispreferred that the fusion protein according to the invention is producedin bacteria.

According to another aspect, the present invention therefore provides anisolated polynucleic acid molecule comprising a nucleic acid sequencewhich encodes a fusion protein according to the invention, or itscomplementary nucleic acid sequence. The polynucleic acid molecule mayalso be useful in further developments of non-natural spidroin proteinsor production systems therefor.

Polynucleic acid molecules according to the invention can be DNAmolecules, including cDNA molecules, or RNA molecules. As the skilledperson is well aware, a nucleic acid sequence may as well be describedby its complementary nucleic acid sequence. Therefore, nucleic acidsequences that are complementary to the nucleic acid sequences accordingto the invention are also encompassed by the protective scope of theinvention.

According to one aspect, the present invention provides a method ofproducing a soluble fusion protein according to the invention. In thefirst step, a polynucleic acid molecule which encodes a fusion proteinaccording to the invention is expressed in a suitable host. In thesecond step, the thus obtained soluble fusion protein in step isisolated, e.g. using chromatography and/or filtration. Optionally, saidsecond step of isolating the soluble fusion protein involves removal ofLPS and other pyrogens.

The present invention further provides a method of producing a majorampullate spidroin protein according to the invention. In the firststep, a polynucleic acid molecule which encodes a fusion proteinaccording to the invention is expressed in a suitable host. In thesecond step, the thus obtained soluble fusion protein is isolated, e.g.using chromatography and/or filtration. In the third step, a solution ofthe isolated fusion protein is provided, and in the fourth step, asuitable cleaving agent is added to the liquid medium. This achievescleavage of the fusion protein at the cleavage agent recognition site,and thereby provides the major ampullate spidroin protein. In anoptional fifth step, the thus obtained major ampullate spidroin proteinis isolated from the liquid medium. Further optionally, said second stepof isolating the soluble fusion protein, and optionally the fifth stepof isolating the major ampullate spidroin protein, involve(s) removal ofLPS and other pyrogens.

The present invention also provides a method of producing a polymer of amajor ampullate spidroin protein according to the invention. In thefirst step, a polynucleic acid molecule which encodes a fusion proteinaccording to the invention is expressed in a suitable host. In thesecond step, the thus obtained soluble fusion protein is isolated, e.g.using chromatography and/or filtration. In the third step, a solution ofthe isolated fusion protein is provided, and in the fourth step, asuitable cleaving agent is added to the liquid medium. This achievescleavage of the fusion protein at the cleavage agent recognition site,and thereby provides the major ampullate spidroin protein. In the fifthstep, the thus obtained major ampullate spidroin protein is allowed topolymerize in the liquid medium. The polymerization typically initiatesat the interface between two different phases, such as liquid/air,liquid/solid, and water/oil interfaces. Thus, this fifth step may alsofurther comprise providing an interface between the liquid medium andanother phase. The other phase is selected from the group consisting ofa gas phase, a liquid phase and a solid phase. As detailed above, theliquid medium is typically an aqueous medium, and suitable other phasesare for instance air and water-immiscible organic solvents, such as oil,e.g. mineral oil suitable for PCR reactions. The presence of theresulting interface stimulates polymerization at the interface or in theregion surrounding the interface, which region extends into the liquidmedium, such that said polymerizing initiates at said interface or insaid interface region. Preferred interfaces include water/air andwater/oil interfaces. Polymerization typically occurs spontaneouslywithin minutes or a few hours, such as within from 1 min to 5 h, uponincubation at room temperature. In an optional sixth step, the thusobtained polymer is isolated from the liquid medium.

In order to obtain a protein with low pyrogenic content, which is anobligate for usage as a biomaterial in vivo, a purification protocoloptimized for removal of lipopolysaccharides (LPS) has been developed.To avoid contamination by released LPS, the producing bacterial cellsare subjected to washing steps with altering CaCl₂ and EDTA. After celllysis, all subsequent purifications steps are performed in lowconductivity buffers in order to minimize hydrophobic interactionsbetween the target protein and LPS. The LPS content is further minimizedby passage of the protein solution through an Endotrap column, which hasa ligand that specifically adsorbs LPS. To assure constant low contentof LPS and other pyrogens, all batches are analyzed using an in vitropyrogen test (IPT) and/or a Limulus amebocyte lysate (LAL) kineticassay. Although produced in a gram-negative bacterial host, therecombinant spidroin proteins can be purified so that residual levels ofLPS and other pyrogens are below the limits required for animal tests,i.e. below 25 EU/implant. In certain embodiments according to theinvention, the content of LPS and other pyrogens in the isolated fusionprotein is 1 EU/mg protein or lower. In certain embodiments according tothe invention, the content of LPS and other pyrogens in the isolatedmajor ampullate spidroin protein is 1 EU/mg protein or lower, preferably0.25 EU/mg protein or lower.

According to another aspect, the present invention provides a polymer ofa major ampullate spidroin protein according to the invention. In apreferred embodiment, the polymer of this protein is obtainable by anyone of the methods therefor according to the invention.

In preferred embodiments, the β-sheet content of the polymer of themajor ampullate spidroin protein according to the invention is above50%, i.e. more than 50% of the secondary structure of the polymer ofthis protein is in β-sheet form. This is advantageous, since it iscontemplated that a higher content of β-sheet structures provides astiffer and/or stronger and/or less extendible fiber.

It is preferable that the polymer of the spidroin protein according tothe invention is a fiber with a macroscopic size, i.e. with a diameterabove 1 μm, preferably above 10 μm and a length above 5 mm. It ispreferred that the fiber has a diameter in the range of 10-400 μm,preferably 60-120 μm, and a length in the range of 0.5-300 cm,preferably 1-100 cm. Other preferred ranges are 0.5-30 cm and 1-20 cm.It is also preferred that the polymer of the spidroin protein accordingto the invention has a tensile strength above 1 MPa, preferably above 2MPa, more preferably 10 MPa or higher. It is preferred that the polymerof the spidroin protein according to the invention has a tensilestrength above 100 MPa, more preferably 200 MPa or higher. The fiber hasthe capacity to remain intact during physical manipulation, i.e. can beused for spinning, weaving, twisting, crocheting and similar procedures.

In other preferred embodiments, the polymer of the spidroin proteinaccording to the invention forms a foam, a gel, a mesh or a film.

According to yet another aspect, the present invention provides a noveluse of a protein fragment comprising a fusion partner and a cleavageagent recognition site for the manufacture of a fusion protein. Thefusion protein is comprising said protein fragment and a spider silkprotein fragment according to the invention, and the two fragments arecoupled via said cleavage agent recognition site. The spider silkprotein fragment preferably consists of from 150 to 420 amino acidresidues.

The present invention will in the following be further illustrated bythe following non-limiting examples.

EXAMPLES Example 1 Cloning and Sequencing of Euprosthenops australisMaSp1 cDNA

The major ampullate glands from approximately 100 adult femaleEuprosthenops australis spiders, collected in South Africa, were used toconstruct a custom-made pDONR222-based CloneMiner cDNA library(Invitrogen, Paisley, UK). cDNA clones encoding the MaSp1 protein wereobtained by screening the library with a cDNA probe encoding an alanine-and glycine-rich fragment originating from Euprosthenops spiders ofunknown subspecies. Colony blotting and detection were performed with anECL direct labelling and detection system (Amersham Biosciences,Uppsala, Sweden) according to the manufacturer's instruction.

One single clone was chosen for further characterization. To obtain fulllength sequence of the cDNA insert from this clone, nested deletionswere made using the Erase-a-Base System (Promega, Southampton, UK), andsequencing was performed on a MegaBase 1000 instrument (AmershamBiosciences).

The resulting 3.8 kb cDNA (SEQ ID NO: 1) encodes a MaSp1 protein (SEQ IDNO: 2) of 1207 amino acid residues, containing a repetitive fragment of34 alanine- and glycine-rich segments (SEQ ID NO: 3), and a C-terminalnon-repetitive fragment of 97 amino acid residues (SEQ ID NO: 4).

Example 2 Sequence Analysis of the Repetitive Fragment of Euprosthenopsaustralis MaSp1 Protein

The repetitive fragment of the Euprosthenops australis MaSp1 proteinsequence of Example 1 (SEQ ID NO: 3) was further analyzed by alignmentof the repetitive segments of the fragment, see FIG. 1. The alignmentwas carefully scrutinized and the following structural information wasconcluded.

The alanine-rich segments of the Euprosthenops australis MaSp1 proteinare 13-15 amino acid residues long and consists of only alanine residuesor all alanine residues but one residue, which is a serine, glutamate orglycine residue.

The repetitive fragment of the Euprosthenops australis MaSp1 proteinfurther contains three related, but distinct, types of glycine-richsegments, c.f. FIG. 2A. Two of the glycine-rich segments differ almostonly in length and occurrence; the most common glycine-rich segmentcontains 23 amino acid residues, while a less abundant variant contains17 amino acid residues. Both of these glycine-rich segments generallylack charged residues or contain one charged residue. In contrast, theshortest glycine-rich segment, containing 14 amino acid residues,uniquely contains the sequence GRGQG (SEQ ID NO: 71) or GQGQG (SEQ IDNO: 72) at the N-terminal end, and GN at the C-terminal end.

The longest glycine-rich segment is represented by the amino acid oneletter consensus sequence GQG(G/S)QGG(Q/Y)GG (L/Q)GQGGYGQGA GSS (SEQ IDNO: 5), and lacks charged residues. It is predicted that this segmentforms coil structures or 3₁-helix structures. The mid-sized glycine-richsegment is represented by the amino acid one letter consensus sequenceGQGGQGQG(G/R)Y GQG(A/S)G(S/G)S (SEQ ID NO: 6), and lacks chargedresidues or contains one charged residue. It is predicted that thissegment forms coil structures. The shortest glycine-rich segment isrepresented by the amino acid one letter consensus sequenceG(R/Q)GQG(G/R)YGQG (A/S/V)GGN (SEQ ID NO: 7). It is predicted that thissegment forms turn structures.

The repetitive fragment of the Euprosthenops australis MaSp1 protein isbuilt up from alternating alanine-rich and glycine rich segments, e.g. .. . A₁G₁A₂G₂A₃G₃A₄G₄A₅G₅ . . . (SEQ ID NO: 56)

It is observed that each of the above-identified shortest and longestglycine-rich segments generally occur as every second glycine-richsegment, e.g. . . .A₁G_(short)A₂G_(long)A₃G_(short)A₄G_(long)A₅G_(short) (SEQ ID NO: 56) .. . .

In contrast, the less abundant, mid-sized glycine-rich fragmentgenerally occurs in between a glycine-rich segment of the longer typeand a glycine-rich segment of the shorter type, e.g. . . .A₁G_(short)A₂G_(long)A₃G_(mid)A₄G_(short)A₅G_(long) . . . (SEQ ID NO:56)

Example 3 Prediction of Secondary and Tertiary Structure of theRepetitive Fragment of Euprosthenops australis MaSp1 Protein

Spidroin polypeptides in solution typically fold by formation of hairpinstructures, which prefigures the anti-parallel β-sheet structure of themature fiber. To discern possible folding patterns for the repetitivefragment (SEQ ID NO: 3) of the Euprosthenops australis MaSp1 protein ofexamples 1-2, protein regions that are compatible with formation ofhairpin or turn structures were identified. The alanine-rich segmentsare unlikely candidates for turn formation since they are predicted toform helical structures, and more importantly, these segments aregenerally held to make up the β-sheets in the fiber.

Using a recently described algorithm for turn predictions (Fuchs, P F &Alix, A J, High accuracy prediction of beta-turns and their types usingpropensities and multiple alignments. Proteins 59, 828-839 (2005)), theshortest glycine-rich segments shows high likelihood for formation oftype II 13-turns, while the two longer glycine-rich segments arepredicted to form coil structures. The high content of GGX triplets inthe longer Gly-rich segments suggests that they can form 3₁-helixstructures.

The repetitive nature of the spidroin amino acid sequences implies anequally repetitive nature of the folding pattern. Taken together, theseobservations result in a folding of the repetitive fragment of theEuprosthenops australis MaSp1 protein as shown in FIG. 2A. It is notablethat the positively charged residues almost invariably are located inthe proposed turn structures.

From the folding pattern of the repetitive fragment of the Euprosthenopsaustralis MaSp1 protein, a motif consisting of alanine-richsegment/(longer) glycine-rich coil segment/alanine-richsegment/(shorter) glycine-rich turn segment/alanine-richsegment/(longer) glycine-rich coil segment/alanine-rich segment, can bediscerned (schematically illustrated in FIG. 2A).

Example 4 Sequence Analysis of the Non-Repetitive C-Terminal Fragment ofMaSp1 Proteins

The primary structure of the C-terminal non-repetitive fragment (SEQ IDNO: 4) of MaSp1 protein from Euprosthenops australis, obtained inExample 1, was aligned with a number of known C-terminal fragments ofMaSp1 and MaSp2 proteins, inter alia from Euprosthenops sp.(Pouchkina-Stantcheva, N N & McQueen-Mason, S J, Molecular studies of anovel dragline silk from a nursery web spider, Euprosthenops sp.(Pisauridae). Comp Biochem Physiol B Biochem Mol Biol 138, 371-376(2004)), Nephila clavipes P19837-5 (Xu, M & Lewis, R V, Structure of aprotein superfiber: spider dragline silk. Proc Natl Acad Sci USA 87,7120-7124 (1990)) and others.

From the alignment shown in FIG. 3, starting from the last Ser in therepetitive fragment, it is evident that the C-terminal regions of MaSp1and MaSp2 are well conserved. Euprosthenops sp and Nephila clavipes have95% identical residues; Euprosthenops australis and Nephila clavipeshave 54% identical residues; and Euprosthenops australis andEuprosthenops sp have 55% identical residues. A consensus sequence ofthe C-terminal regions of MaSp1 and MaSp2 is provided as SEQ ID NO: 8.In FIG. 3, the following MaSp proteins are aligned, denoted with GenBankaccession entries where applicable:

Species and spidroin protein Entry Euprosthenops sp MaSp1 (Pouchkina-Cthyb_Esp Stantcheva, NN & McQueen-Mason, SJ, ibid) Euprosthenopsaustralis MaSp1 (SEQ ID NO: 4) CTnat_Eau Argiope trifasciata MaSp1AF350266_At1 Cyrtophora moluccensis Sp1 AY666062_Cm1 Latrodectusgeometricus MaSp1 AF350273_Lg1 Latrodectus hesperus MaSp1 AY953074_Lh1Macrothele holsti Sp1 AY666068_Mh1 Nephila clavipes MaSp1 U20329_Nc1Nephila pilipes MaSp1 AY666076_Np1 Nephila madagascariensis MaSp1AF350277_Nm1 Nephila senegalensis MaSp1 AF350279_Ns1 Octonoba variansSp1 AY666057_Ov1 Psechrus sinensis Sp1 AY666064_Ps1 Tetragnathakauaiensis MaSp1 AF350285_Tk1 Tetragnatha versicolor MaSp1 AF350286_Tv1Araneus bicentenaries Sp2 ABU20328_Ab2 Argiope amoena MaSp2AY365016_Aam2 Argiope aurantia MaSp2 AF350263_Aau2 Argiope trifasciataMaSp2 AF350267_At2 Gasteracantha mammosa MaSp2 AF350272_Gm2 Latrodectusgeometricus MaSp2 AF350275_Lg2 Latrodectus hesperus MaSp2 AY953075_Lh2Nephila clavipes MaSp2 AY654293_Nc2 Nephila madagascariensis MaSp2AF350278_Nm2 Nephila senegalensis MaSp2 AF350280_Ns2 Dolomedestenebrosus Fb1 AF350269_DtFb1 Dolomedes tenebrosus Fb2 AF350270_DtFb2Araneus diadematus ADF−1 U47853_ADF1 Araneus diadematus ADF−2U47854_ADF2 Araneus diadematus ADF−3 U47855_ADF3 Araneus diadematusADF−4 U47856_ADF4

Example 5 Construction of MaSp1 Genes

A DNA sequence encoding the Euprosthenops australis-derived protein5Gly/Ala-CT_(nat) (SEQ ID NO: 9) was amplified by PCR with an AdvantageGC2 kit (BD Biosciences, San Jose, Calif., USA), using a MaSp1 clonefrom the cDNA library of Example 1 as template. Restriction enzymerecognition sites BamHI and HindIII were introduced at the 5′- and3′-ends, respectively, and a stop codon was introduced upstream of theHindIII site, by use of designed primers. TheBamHI-5Gly/Ala-CT_(nat)-HindIII construct was then subcloned into amodified pET32 vector (Merck Biosciences, Darmstadt, Germany), preparedas described in Example 6(C) below.

Example 6 Construction of Chimeric MaSp1 Genes

(A) REP Gene Fragments

DNA sequences coding for partial repetitive fragments (REP) denoted3Gly/Ala and 4Gly/Ala were amplified by PCR with LA Taq (TaKaRa Bio;Saint Germain-en-laye, France) in the presence of betaine (Henke W etal, Betaine improves the PCR amplification of GC-rich DNA sequences.Nucleic Acids Res 25, 3957-3958 (1997)), using a partial cDNA cloneencoding a repetitive region of Euprosthenops sp MaSp1 protein(Pouchkina-Stantcheva, N N & McQueen-Mason, S J, Molecular studies of anovel dragline silk from a nursery web spider, Euprosthenops sp.(Pisauridae). Comp Biochem Physiol B Biochem Mol Biol 138, 371-376(2004)) (GenBank entry CQ974358 or CQ816656) as template. Restrictionenzyme recognition sites were introduced at the 5′- and 3′-ends, givingthe following constructs: NcoI-3Gly/Ala-NheI and NcoI-4Gly/Ala-NheI tobe joined with a CT fragment (see below); and a NcoI-4Gly/Ala-XhoI cloneto be individually expressed, where a stop codon was inserted directlyupstream of the XhoI site.

(B) CT Gene Fragments

A DNA sequence coding for the non-repetitive C-terminal domain fromEuprosthenops sp (but with a high degree of similarity to MaSp1 fromNephila clavipes and Nephila senegalensis) was amplified by PCR using agenomic DNA clone encoding a C-terminal MaSp1 domain(Pouchkina-Stantcheva, N N & McQueen-Mason, S J, Molecular studies of anovel dragline silk from a nursery web spider, Euprosthenops sp.(Pisauridae). Comp Biochem Physiol B Biochem Mol Biol 138, 371-376(2004)). Restriction enzyme recognition sites were introduced at the 5′-and 3′-ends, giving NheI-2Gly/Ala-CT_(hyb)-XhoI, to be joined with the3Gly/Ala and 4Gly/Ala partial REP clones (see above), andNcoI-2Gly/Ala-CT_(hyb)-XhoI, to be individually expressed.

(C) Construction of REP-CT Hybrid MaSp1 Genes

The 3Gly/Ala and 4Gly/Ala REP clones were joined with the CT clonesusing the pCR® 2.1-TOPO® vector (Invitrogen). Then, the resulting, fused5Gly/Ala-CT_(hyb) and 6Gly/Ala-CT_(hyb) clones were excised with NcoIand XhoI, and subcloned into a modified pET32 vector (Novagen), wherethe original thrombin cleavage site was removed and a new thrombin sitewas introduced downstream of the enterokinase cleavage site.

Example 7 Expression of MaSp1 Fusion Proteins

The MaSp1 proteins coded for by the genes constructed in examples 5-6were expressed as fusion proteins (of the type X-REP-CT) as follows,using a modified pET32vector: thioredoxin-tag/His-tag/S-tag/thrombincleavage site/MaSp1 gene, encoding a thioredoxin/His₆/S-tag/thrombincleavage site/MaSp1 protein, and an ampicillin resistance gene undercontrol of the T7 promoter.

The different MaSp1 constructs in pET32 expression vectors weretransformed into Escherichia coli BL21(DE3) cells (Merck Biosciences).The cells were grown at 30° C. in Luria-Bertani medium containingampicillin to an OD₆₀₀ of 1.0-1.5, induced with IPTG and furtherincubated for 4 h at room temperature. The cells were harvested bycentrifugation, and lysed by DNAseI and lysozyme in 20 mM Tris-HCl, pH8.0, 20 mM imidazole, with 0.5 M NaCl, and further purified by His-tagaffinity chromatography on Ni-NTA agarose (Qiagen, West Sussex, UK).Bound fusion proteins were eluted from the Ni-NTA column with 200 mMimidazole in 20 mM Tris-HCl, pH 8.0, with 0.5 M NaCl, and dialyzedagainst 20 mM Tris-HCl, pH 8.0. The resulting fusion proteins were >90%pure as judged by coomassie-stained SDS polyacrylamide gels and solublein 20 mM Tris-HCl, pH 8.0. This process yielded approximately 40 mg/lculture of fusion protein, which was stable for weeks withoutsignificant precipitation.

In another experiment, the fusion proteins were expressed asHis₆/thioredoxin/His₆/thrombin cleavage site/MaSp1 proteins from aplasmid containing the corresponding gene and a kanamycin resistancegene under control of the T7 promoter.

Example 8 Formation of Fibers from MaSp1 Proteins

Cleavage of the tags from the fusion proteins resulting from Example 7,was performed in 20 mM Tris-HCl, pH 8, with a thrombin:fusion proteinratio of 1:1000 (w/w), under very gentle rocking at room temperature.Thrombin cleavage was complete within 30-60 min, as judged by SDS-PAGE.The resulting MaSp1 proteins (FIG. 2B, SEQ ID NOS: 9-13) spontaneouslypolymerized into macroscopic fibers to varying extents, see Table 1. Thefibers were initially formed at the water/air interface. The formationcould be observed by the naked eye from about 1 hour of incubation (seeFIG. 4A, 4B), and after about 5 hours occurred no further fiber growth.6Gly/Ala-CT_(hyb) fibers were up to approximately 2 cm long, and5Gly/Ala-CT_(nat) fibers were ≧10 cm long. Repeated experiments yielded5Gly/Ala-CT_(nat) fibers that were ≧20 cm (see FIG. 4C), and even ≧2 mlong. Fiber formation could be observed by the naked eye from about 10min of incubation.

Fibers were isolated and washed with buffer and thereafter subjected toN-terminal amino acid sequence analysis, which showed only the sequenceof the MaSp1 protein. This shows that the cleaved tags are absent in thefibers.

Example 9 Analysis of MaSp1 Protein Fibers

A. Tensile Strength Measurements

The tensile strength of the 6Gly/Ala-CT_(hyb) (SEQ ID NO: 13) and5Gly/Ala-CT_(nat) (SEQ ID NO: 9) fibers of example 8 was determined asfollows. In order to handle the shorter (1-2 cm) 6Gly/Ala-CT_(hyb)fibers for tensile strength measurements, they were incubated shortly in15% glycerol in water before they were air-dried. The longer (10 cm)5Gly/Ala-CT_(nat) fibers were either untreated, incubated shortly in 15%glycerol, or drawn by hand in 75% methanol before air-drying. Tensilestrength of air-dried fibers was measured by pulling the fibers in aZwick Material Tester at a rate of 10 mm/min. See Table 1.

The tensile strength of glycerol-treated air-dried 1-2 cm long fibersfrom 6Gly/Ala-CT_(hyb) (SEQ ID NO: 13) was about 2 MPa, and the strengthof 10 cm fibers from 5Gly/Ala-CT_(nat) (SEQ ID NO: 9) was 4-5 MPa. Tencm long 5Gly/Ala-CT_(nat) fibers drawn in the dehydrating solventmethanol before air-drying displayed a tensile strength of 2-3 MPa,which is slightly less than for glycerol-treated fibers of the sametype. The highest tensile strength now measured was 10 MPa, which wasfound for an air-dried 10 cm long 5Gly/Ala-CT_(nat) fiber withoutfurther treatment.

The range of tensile strengths (2-10 MPa) is comparable to the lowervalues reported for regenerated spider silk fibers (2-320 MPa). Thelongest spontaneously formed fibers derive from the 5Gly/Ala-CT_(nat)construct, and such air-dried fibers also show the greatest tensilestrength. Potentially, this could be due to its 12-15 residues longpoly-Ala segments, relative the 8-14 residue Ala segments in6Gly/Ala-CT_(hyb), which would give a greater proportion of crystallineβ-sheet conformation in the former protein.

TABLE 1 Fiber forming capacity of MaSp1 proteins SEQ Fiber Fiber Fibertensile ID forming length strength Protein NO capacity (cm) (MPa)5Gly/Ala-CT_(nat) 9 ++++ ≧10 2-10 5Gly/Ala-CT_(nat) 9 +++++ ≧20 180-230(Example 11) 4Gly/Ala 10 aggregates n.a. n.a. 2Gly/Ala-CT_(hyb) 11aggregates n.a. n.a. 5Gly/Ala-CT_(hyb) 12 + ≦1 n.d. 6Gly/Ala-CT_(hyb) 13+++ 1-2 ≦2 n.a. = not applicable n.d. = not determinedB. Scanning Electron Microscopy

The microscopic architecture of the 6Gly/Ala-CT_(hyb) and5Gly/Ala-CT_(nat) fibers was analyzed with scanning electron microscopy(SEM) (FIG. 5). Briefly, samples were applied on SEM-stubs andvacuum-coated with a 6 nm layer of gold and palladium. Specimens wereobserved and photographed in a LEO 1550 FEG SEM using an accelerationvoltage of 10 kV.

This revealed diameters of 10-30 μm for single fibers, with individualfibers displaying rather homogenous diameters (FIG. 5 a showing6Gly/Ala-CT_(hyb), SEQ ID NO: 13). In addition to the macroscopicfibers, gel-like particles were found. After air-drying such particlesof 6Gly/Ala-CT_(hyb) directly on a SEM-stub, fibers approximately 10-15μm in diameter were seen (FIG. 5b, c). The diameter of macroscopicfibers of 5Gly/Ala-CT_(nat) (SEQ ID NO: 9), drawn in 75% methanol andair-dried, were 60-120 μm and they apparently contain several alignedfibers (FIG. 5d-f). Fiber twisted before air-drying (FIG. 5e), end offiber (FIG. 5f).

C. Circular Dichroism Spectroscopy

Fibers consisting of 6Gly/Ala-CT_(hyb) protein (SEQ ID NO: 13) or5Gly/Ala-CT_(nat) (SEQ ID NO: 9), prepared in Example 8, were washed in20 mM phosphate buffer, pH 7, and suspended in 2% SDS in the samebuffer. Circular Dichroism spectra from 250 to 190 nm were recorded at22° C. in a 0.1 cm path length quartz cuvette, using a Jasco J-810spectropolarimeter. The scan speed was 50 nm/min, response time 2 sec,acquisition interval 0.1 nm, and the band width 1 nm.

The spectrum shown in FIG. 6 is an accumulation of three scans of fibersof 6Gly/Ala-CT_(hyb) protein (SEQ ID NO: 13). It displays a minimum at220 nm and a maximum at 195 nm, features that are characteristic ofantiparallel β-sheet structures. Highly similar spectra were obtainedfor fibers of 5Gly/Ala-CT_(nat) (not shown). The spontaneously formedfibers thus exhibit similar morphology and structure as native andregenerated spider silk fibers.

Example 10 Biocompatibility of Recombinant Spider Silk

Since it is desirable to use spider silk fibers in biomedicalapplications, the biocompatibility of the fibers has been evaluated byan investigation of effects of recombinantly produced silk using twodifferent cell types.

The MaSp1 protein 5Gly/Ala-CT_(nat) (SEQ ID NO: 9) was expressed inbacteria as described in examples 7-8. Purified protein was used toproduce artificial silk fibers with lengths of >10 cm, and even >20-200cm, and diameters of around 100 μm.

A. Embryonal Mouse Mast Cells

Embryonal (day 12.5) mouse mast cells (in vitro proliferated for eightweeks using IL-8 and mast stem cell factor) were seeded at two differentcell densities, the higher density being about four times the lowerdensity. These cells do not adhere to the plastic surface, but grow insuspension. Pieces of the silk fiber, each about 0.5 cm long, were addedto the wells. Mast cells were incubated for three days, with or withoutthe presence of silk fiber, and thereafter living and dead cells werecounted after staining with Trypan blue (FIG. 7). The bars show the meanvalues with standard error mean, n=2, each sample is counted intriplicate.

The mast cells are not affected by the presence of the silk fibers.After three days of growth, there are no significant differences in celldeath or proliferation compared to the negative controls grown withoutsilk fibers.

B. Human Embryonic Kidney (HEK) 293 Cells

Pieces of the silk fiber, about 0.5 cm long, were adsorbed to the bottomof 6-well microtiter plates by letting them dry from a small volume ofbuffer. The fibers do not detach when cell growth media is added. Humanembryonic kidney (HEK) 293 cells were then plated at different celldensities and allowed to grow for a total of six days. The HEK-293 cellsadhere and grow attached to the plastic cell surface. The ability of theHEK-293 cells to grow in the proximity of the fibers, and the physicalattachment of the cells to the fibers was studied.

The HEK-293 cells attached and proliferated normally in the wellscontaining silk fibers (as observed under the light microscope). Thecells grew very closely along the fiber edges, and apparently even grewunder a partly detached fiber (FIG. 8). After seven days, the fiberswere carefully detached from the plastic surface, and it was clearlyseen that groups of cells were physically attached to the fibers. Thefiber covers the upper right half of the figure. HEK293-cells are seenattached to the edge of the fiber, and also grow under the fiber.

The two different cell types (mast cells, HEK-293) studied were notaffected by the presence of recombinant silk fibers, even atcomparatively high amounts of silk. This indicates that the testedartificial silk fibers resemble wild type dragline silk of Euprosthenopsaustralis, in being non-toxic and biocompatible. The artificial silkfibers thus appear suitable for biomedical applications.

Example 11 Mechanical Properties and Structure of MaSp1 Protein Fibers

The mechanical properties of fibers from 5Gly/Ala-CT_(nat) (SEQ ID NO:9) were examined using tensile tests performed to yield stress-straincurves (FIG. 9). The tensile properties were characterized using a ZwickRoell Z2.5 material tester (Zwick, Ulm, Germany). The tests wereperformed in air at ambient conditions (20° C. and 52% relativehumidity) using a loading speed of 10 mm/min. Fiber pieces weretransferred directly from buffer, mounted and subjected to twostretching-relaxation cycles. In order to generate a homogenous silkthread suitable for tensile testing, the fibers were elongated usingstretch-relaxation cycles. First, the fibers were elongated by pullingup to a force of 0.1 N. After relaxation, they were further drawn untila force of 0.25 N was applied.

This treatment generated elongated homogenous fibers with a diameter ofapproximately 80 μm as determined by height measurements using aMitutoyo IDC-112B instrument (Mitutoyo Corp, Tokyo, Japan) and confirmedby scanning electron microscopy (SEM) as follows. Before and afterstretch-relaxation cycles, fiber pieces were applied on SEM stubs andair-dried overnight. The samples were vacuum-coated with a 6 nm layer ofgold and palladium. Specimens were observed and photographed with a LEO1550 FEG microscope (Carl Zeiss, Oberkochen, Germany) using anacceleration voltage of 10 kV.

The drawn fibers were cut into pieces, the ends of which were fixedbetween cardboard paper with glue (Loctite 420, Loctite, Göteborg,Sweden). Fiber samples were then fixed in the grips of the materialtester and stretched until they broke. Stress-strain curves wereconstructed using the initial cross-sectional area of the pre-drawnfibre, assuming a circular cross-section. The stress values arenormalized to the initial cross-sectional area of the fiber. The straincorresponds to dL/L₀ where L₀ is the initial length of the fiber and dLis the change in fiber length. In FIG. 9, the stress-strain curves forthree different samples of double drawn fibers of 5Gly/Ala-CT_(nat) (SEQID NO: 9) are shown, and their tensile strength measured approximately0.2 GPa.

The microscopic architecture of the fibers was analyzed by SEM (FIG.10). The spontaneously formed fibers have a homogenous flattenedappearance and a width of up to several hundred micrometers, while theheight measures some ten micrometers (FIG. 10 a,b).

After the fibers had been subjected to stretch-relaxation cycles, theircross section adopted a more rounded shape with a compact substructureof tightly aligned fibrils (FIG. 10 c-f). The appearance of cut orfractured surfaces (FIG. 10 e,f) further attest to the compactness ofthe produced fiber.

In conclusion, the spontaneously formed fibers show similar morphologyand mechanical properties as native or regenerated spider silk fibers,even without spinning.

Example 12 Spidroin Protein Variants

Strong intermolecular interactions are thought to contribute to theimpressive tensile strength of spider silk. Therefore, variants ofminiature spidroins that allow intermolecular covalent cross-linking inthe fibers have been produced. Two different mutant spidroin proteinshave been constructed by site-directed mutagenesis to introduce twocysteine residues in the first (SEQ ID NO: 14, positions 36 and 37) andthe fourth (SEQ ID NO: 15, positions 128 and 129) alanine block,respectively. These variants have been expressed and isolated using thesame protocol as described in Examples 7-8 for the genes constructed inExamples 5-6.

These variants (SEQ ID NOS: 14-15) form fibers in the same manner as5Gly/Ala-CT_(nat) (SEQ ID NO: 9).

In order to elucidate the importance of dimerization of the C-terminaldomain, a variant where the cysteine residue in the C-terminal domain isexchanged for a serine residue has been constructed (SEQ ID NO: 16,position 222). However, this variant (SEQ ID NO: 16) forms fibers in thesame manner as 5Gly/Ala-CT_(nat) (SEQ ID NO: 9).

Example 13 Removal of LPS and Other Pyrogens from Expressed SpidroinProteins

E. coli cells expressing the desired spidroin fusion protein are washedwith the following buffers:

A: 100 mM Tris, pH 8,

B: 5 mM CaCl₂, 100 mM Tris, pH 8,

C: 10 mM EDTA, 100 mM Tris, pH 8,

D: 100 mM Tris, pH 8, and

E: 100 mM Tris, pH 8.

Thereafter, the cells are lysed in 20 mM Tris, pH 8 supplemented withlysozyme and DNaseI. The protein sample is then loaded on a Ni-sepharosematrix and washed with 20 mM Tris, 10-100 mM imidazole, pH 8 beforeelution with 20 mM Tris, 100-300 mM imidazole, pH 8. Relevant fractionsare pooled and dialyzed against 20 mM Tris, pH 8 over night. The proteinsample is then supplemented with 100 μM CaCl₂ and finally passed throughan EndoTrap Blue column, previously equilibrated with 20 mM Tris, 100 μMCaCl₂, pH 8. In this way, protein samples with a pyrogen content of 1EU/mg protein can be obtained, as judged by IPT and a LAL kinetic assay.

The fusion protein is then proteolytically cleaved with thrombin using a1:1000 (w/w) thrombin:fusion protein ratio, which induces fiberformation (as described above). The fibers are washed 3 times in 20 mMTris, pH 8 and finally 3 times in water. This gives fibers with apyrogen content of 0.25 EU/mg fiber.

The structural characteristics of the fibers are unaffected afterautoclaving at 125° C. and 1.5 bar for 10 min, which enables efficientsterilization of the material. The fibers are chemically stable and cannot be solubilized in either of 8 M urea, 6 M GuaHCl, or neat HAc.However, the fibers can be solubilized in neat HFIP or formic acid.

The invention claimed is:
 1. A support for cell adherence and growth,comprising a polymer of a major ampullate spidroin protein, which majorampullate spidroin protein is consisting of from 150 to 420 amino acidresidues and being defined by the formula REP-CT, wherein REP is aprotein fragment having from 80 to 300 amino acid residues, wherein saidfragment is selected from the group of L(AG)_(n)L, L(AG)_(n)AL,L(GA)_(n)L, and L(GA)_(n)GL, wherein n is an integer from 4 to 8; eachindividual A segment is an amino acid sequence of from 8 to 18 aminoacid residues, wherein from 0 to 3 of the amino acid residues are notAla, and the remaining amino acid residues are Ala; each individual Gsegment is an amino acid sequence of from 12 to 30 amino acid residues,wherein at least 40% of the amino acid residues are Gly; and eachindividual L segment is a linker amino acid sequence of from 0 to 20amino acid residues; and CT is a protein fragment having from 70 to 120amino acid residues and at least 80% identity to an amino acid sequenceselected from the group consisting of SEQ ID NO: 4, amino acid residues172-269 of SEQ ID NO: 9, and amino acid residues 172-269 of SEQ ID NO:16.
 2. The support for cell adherence and growth according to claim 1,wherein the polymer has a shape selected from the group consisting of afiber, a foam, a gel, a mesh, and a film.
 3. The support for celladherence and growth according to claim 1, wherein n is an integer from4 to
 6. 4. The support for cell adherence and growth according to claim3, wherein n is an integer from 4 to
 5. 5. The support for celladherence and growth according to claim 1, wherein each individual Asegment is, or has at least 80% identity to, an amino acid sequenceselected from the group of amino acid residues 7-19, 43-56, 71-83,107-120, 135-147, 171-183, 198-211, 235-248, 266-279, 294-306, 330-342,357-370, 394-406, 421-434, 458-470, 489-502, 517-529, 553-566, 581-594,618-630, 648-661, 676-688, 712-725, 740-752, 776-789, 804-816, 840-853,868-880, 904-917, 932-945, 969-981, 999-1013, 1028-1042 and 1060-1073 ofSEQ ID NO: 3; amino acid residues 31-42, 61-75, 90-104, 122-135 and153-171 of SEQ ID NO: 9; amino acid residues 12-25, 46-60, 75-88,112-119, 150-158 and 173-180 of SEQ ID NO: 13; amino acid residues 31-42of SEQ ID NO: 14; and amino acid residues 122-135 of SEQ ID NO: 15; andeach individual G segment is, or has at least 80% identity to, an aminoacid sequence selected from the group of amino acid residues 20-42,57-70, 84-106, 121-134, 148-170, 184-197, 212-234, 249-265, 280-293,307-329, 343-356, 371-393, 407-420, 435-457, 471-488, 503-516, 530-552,567-580, 595-617, 631-647, 662-675, 689-711, 726-739, 753-775, 790-803,817-839, 854-867, 881-903, 918-931, 946-968, 982-998, 1014-1027,1043-1059 and 1074-1092 of SEQ ID NO: 3; SEQ ID NOS: 5-7; amino acidresidues 11-30, 43-60, 76-89, 105-121 and 136-152 of SEQ ID NO: 9; andamino acid residues 1-11, 26-45, 61-74, 89-111, 120-149 and 159-172 ofSEQ ID NO:
 13. 6. The support for cell adherence and growth according toclaim 1, wherein said CT fragment has at least 90% identity to an aminoacid sequence selected from the group consisting of SEQ ID NO: 4, aminoacid residues 172-269 of SEQ ID NO: 9, and amino acid residues 172-269of SEQ ID NO:
 16. 7. The support for cell adherence and growth accordingto claim 6, wherein said CT fragment has at least 95% identity to anamino acid sequence selected from the group consisting of SEQ ID NO: 4,amino acid residues 172-269 of SEQ ID NO: 9, and amino acid residues172-269 of SEQ ID NO:
 16. 8. The support for cell adherence and growthaccording to claim 7, wherein said CT fragment is an amino acid sequenceselected from the group consisting of SEQ ID NO: 4, amino acid residues172-269 of SEQ ID NO: 9, and amino acid residues 172-269 of SEQ ID NO:16.
 9. The support for cell adherence and growth according to claim 1,wherein the major ampullate spidroin protein has at least 80% identityto amino acid residues 11-269 of SEQ ID NO:
 9. 10. The support for celladherence and growth according to claim 9, wherein the major ampullatespidroin protein has at least 90% identity to amino acid residues 11-269of SEQ ID NO:
 9. 11. The support for cell adherence and growth accordingto claim 10, wherein the major ampullate spidroin protein has at least95% identity to amino acid residues 11-269 of SEQ ID NO:
 9. 12. Thesupport for cell adherence and growth according to claim 1, wherein themajor ampullate spidroin protein is selected from SEQ ID NOS: 9 and12-16.
 13. The support for cell adherence and growth according to claim12, wherein the major ampullate spidroin protein is SEQ ID NO:
 9. 14.The support for cell adherence and growth according to claim 1, whereinthe content of lipopolysaccharide (LPS) and other pyrogens is 1 EU/mg ofisolated protein or lower.
 15. A method of culturing eukaryotic cells,comprising: growing said eukaryotic cells attached to the support forcell adherence and growth according to claim
 1. 16. A medical product,comprising: the support for cell adherence and growth according toclaim
 1. 17. The medical product according to claim 16, wherein saidmedical product is selected from the group consisting of implants, woundclosure systems, band-aids, sutures, wound dressings and scaffolds fortissue engineering and guided cell regeneration.
 18. The medical productaccording to claim 17, wherein said medical product is an implant. 19.The medical product according to claim 17, wherein said medical productis selected from the group consisting of scaffolds for tissueengineering and guided cell regeneration.
 20. The medical productaccording to claim 16, further comprising eukaryotic cells adhered toand growing on said support.